The object of this proposal is to study the regulation and the bioactivity of the A-protein (Mr 94,000) and B-protein (Mr 120,000) of human breast cancer progesterone receptors (PR). We will use progestin responsive breast cancer cell lines, in situ photoaffinity labeling, anti-receptor antibodies, and PR cDNA probes to: a) study the transcriptional and/or post- transcriptional regulation of PR messages by progestins and estradiol, and assess hormonal effects on mRNA and protein turnover rates; b) determine PR gene copy number in normal and breast cancer cells where PR are over-expressed; c) analyze the processing state and the 5'- and 3'-termini of six different PR message bands; d) show, by hybrid selection and hybrid arrest translation, whether only B-receptors or both A- and B-receptors can be synthesized in vitro; d) transfect full length wild-type PR cDNA, and cDNAs in which A- or B-receptor translation start sites have been destroyed by mutation, or together, are needed for response to progestins; f) develop methods to study expression, by isolating PR-regulated messages, and by searching for progestin-regulated growth inhibitory factors in cell-conditioned media; g) address the clinical applications of some of these studies.